In many mixed-signal integrated circuits, a bandgap voltage reference is used. This bandgap voltage may show interferences. For suppressing the interferences, normally an internal resistor 11 and an external filter capacitor 13 is used, as depicted in FIG. 1. For this purpose, the respective chip comprises a pad in order to allow the external filter capacitor 13 of suitable capacitance to be connected to the chip. The voltage at the output node 14 is filtered and serves in the present example as input signal of a plurality of buffers 12.
Another circuit, which needs filtering, is illustrated in FIG. 2. It divides the power supply voltage VDD, for instance to half its value, and is then used for controlling the optimum working point of output stages 22. The dividing of the power supply voltage VDD is done by means of two resistors 21. One may also employ two CMOS transistors instead. Also in this circuit an external filter capacitor 23 is used, as depicted in FIG. 2.
Yet another circuit, where filtering of the power supply voltage VDD provided by a bandgap voltage source 30 is required, is depicted in FIG. 3. In this circuit an external capacitor 33 is required, too. A programmable gain stage 32 is employed which amplifies the reference voltage provided by the bandgap voltage source 30 and filtered by the external capacitor 33. The output of the gain stage 32 can be used for the same purpose as in FIG. 2.
These state-of-the-art systems have drawbacks. The external components of FIGS. 1 through 3 add pads to the respective chips and add pins to the chip packages. Furthermore, these approaches add area to the print and cost to the bill of materials. It is a disadvantage of the circuit depicted in FIG. 3, that the gain stage 32 must be programmed by software in conjunction with measurement of the voltage provided by a battery. This needs a simple routine. If not available, a standard setting will be chosen which does not guarantee an optimum output stage working point at extreme supply voltages.
The above summarizes state of the art approaches being described in the technical literature and in related patents. In the U.S. Pat. No. 6,657,481 specific current mirror circuits are proposed. The circuits disclosed in this patent just comprise PMOS transistors. The main drawback of such a circuit, if used in a filter structure, is that the circuit is only able to charge the capacitor. This will lead to a rising voltage on the capacitor if interferences are present. The U.S. Pat. No. 6,657,481 does not offer a solution for filtering a reference voltage.
Thus, it would be generally desirable to provide a solution allowing on-chip filtering of a reference voltage. Furthermore, there is a demand for special circuits that can be employed in a mobile phone where the supply voltage, as supplied by a battery, fluctuates if the mobile phone is in a transmit mode.
It is thus an object of the present invention to provide an apparatus for improved filtering of a supply voltage that preferably does not require an external capacitor., It is a further objective of the present invention to provide an apparatus for improved filtering of a supply voltage for employment in battery comprising mobile phone.